A solar battery is greatly expected to serve as a clean renewable energy source, and researches have been conducted for practical application of a monocrystalline-silicon, polycrystalline-silicon or amorphous-silicon-based solar battery and a solar battery containing a compound such as cadmium telluride and indium copper selenide. For the spread of solar battery as a household power source, however, any of these batteries faces many problems to be overcome, including a higher production cost, difficulty in ensuring raw materials, difficulty in recycling, and difficulty in realizing a larger area. Accordingly, there have been proposed solar batteries using an organic material in an attempt to achieve a larger area and a lower cost. However, any of these batteries has a conversion efficiency of about 1%, which falls very short of practical use.
Under such circumstances, Graetzel et al. disclosed a photoelectric conversion element and a solar battery using semiconductor particles sensitized by a dye, as well as materials and production technique needed to produce this solar battery in 1991 (see, for example, Non-patent document 1 and Patent document 1). This battery is a wet solar battery comprising a porous titania thin film sensitized by a ruthenium dye as a working electrode. This solar battery has the advantages that the photoelectric conversion element can be provided at a low cost because inexpensive materials can be used without highly purification, and that the solar battery can convert solar light into electricity over a wide visible light wavelength range because a dye having broad absorption band is used. However, the conversion efficiency must be further improved for practical use. Thus, there is a need for a dye which has a higher absorption coefficient and absorb light of longer wavelength.
Patent document 2 discloses a mononuclear metal complex containing a dipyridyl ligand, which is a metal complex dye useful for a photoelectric conversion element. In addition, Non-patent document 2 discloses a polynuclear β-diketonate complex dye.
Meanwhile, Patent document 3 discloses a polynuclear complex containing a plurality of metals and a plurality of ligands wherein a bridging ligand (BL) coordinating to the plurality of metals has both a coordination structure with a conjugated heterocyclic ring and a coordination structure without a conjugated heterocyclic ring, which is regarded as a novel polynuclear complex having the excellent photoelectric conversion function of emitting electrons while receiving energy from active ray such as light.
Patent document 4 discloses a binuclear metal complex having a coordination structure with a conjugated heterocyclic ring, which is a metal complex dye for realizing a photoelectric conversion element having higher photoelectric conversion efficiency.
In the case of a ruthenium-osmium complex dye, in particular, its ability to absorb light in the longer wavelength region must be further improved for practical use. However, in a ruthenium-osmium complex dye which absorb light of longer wavelength, the HOMO-LUMO of the dye is apt to mismatch the energy level of TiO2 (CB) or iodine redox, commonly-used peripheral members [semiconductor particle and electrolyte (redox couple)], and therefore electron transfer may not effectively occur. Thus, there is a need for a ruthenium-osmium complex dye, which is capable of absorbing light of longer wavelength and has an energy level matched with a peripheral member.